Get ready for tox 4

Changes:
- eliminate whitespace in passenv values
- account for stricter allowlist checking
- removed skipsdist=True, which in tox 4 appears to prevent
  rbd-iscsi-client from being installed in the testenvs
- updated the 'bindep' testenv so it would work if we had
  a bindep.txt, which we currently don't

We can't make tox 4 the default because tox 4 only supports python
>=3.7, and rbd-iscsi-client still supports py3.6.  When our CI job
for py3.6 lands on a system whose default python is 3.6, the job
would fail because it couldn't install tox 4.

Change-Id: I651f762bf6aac9679d1fd74817e7e5f201ced774

tox.ini

@@ -1,9 +1,9 @@
 [tox]
 minversion = 3.18.0
-requires = virtualenv>=20.4.2
-# python runtimes: https://governance.openstack.org/tc/reference/project-testing-interface.html#tested-runtimes
+# specify virtualenv here to keep local runs consistent with the
+# gate (it sets the versions of pip, setuptools, and wheel)
+requires = virtualenv>=20.17.1
 envlist = py3,pep8
-skipsdist = True
 # this allows tox to infer the base python from the environment name
 # and override any basepython configured in this file
 ignore_basepython_conflict=true
@@ -30,7 +30,13 @@ commands =
 
 allowlist_externals = bash
                       find
-passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
+passenv =
+    http_proxy
+    HTTP_PROXY
+    https_proxy
+    HTTPS_PROXY
+    no_proxy
+    NO_PROXY
 
 [testenv:py36]
 # We have to override the tox default py36 testenv because we need to use
@@ -51,6 +57,8 @@ commands =
 [testenv:fast8]
 # Use same environment directory as pep8 env to save space and install time
 envdir = {toxworkdir}/pep8
+allowlist_externals =
+  {toxinidir}/tools/fast8.sh
 commands =
   {toxinidir}/tools/fast8.sh
 
@@ -135,8 +143,8 @@ import_exceptions = rbd_iscsi_client.i18n
 # system dependencies are missing, since it's used to tell you what system
 # dependencies are missing! This also means that bindep must be installed
 # separately, outside of the requirements files, and develop mode disabled
-# explicitly to avoid unnecessarily installing the checked-out repo too (this
-# further relies on "tox.skipsdist = True" above).
+# explicitly to avoid unnecessarily installing the checked-out repo too
+skip_install = True
 deps = bindep
 commands = bindep test
 usedevelop = False